1. Technical Field of the Invention
This invention relates to optical lithography and more particularly to direct write optical lithography.
2. Description of the Related Art
Polymer arrays, such as the GeneChip(copyright) probe array (Affymetrix, Inc., Santa Clara, Calif.), can be synthesized using light-directed methods described, for example, in U.S. Pat. Nos. 5,143,854; 5,424,186; 5,510,270; 5,800,992; 5,445,934; 5,744,305; 5,384,261 and 5,677,195 and PCT published application no. WO 95/11995, which are hereby incorporated by reference in their entireties. As an example, light-directed synthesis of oligonucleotides employs 5xe2x80x2-protected nucleosidephosphoramidite xe2x80x9cbuilding blocks.xe2x80x9d The 5xe2x80x2-protecting groups may be either photolabile or acid-labile. A plurality of polymer sequences in predefined regions are synthesized by repeated cycles of deprotection (selective removal of the protective group) and coupling. Coupling (i.e., nucleotide or monomer addition) occurs only at sites that have been deprotected. Three methods of light-directed synthesis are: use of photolabile protecting groups and direct photodeprotection (DPD); use of acid-labile 4,4xe2x80x2-dimethoxytrityl (DMT) protecting groups and a photoresist; use of DMT protecting groups and a polymer film that contains a photoacid generator (PAG).
These methods have many process steps similar to those used in semiconductor integrated circuit manufacturing. These methods also often involve the use of photomasks (masks) that have a predefined image pattern which permits the light used for synthesis of the polymer arrays to reach certain regions of a substrate but not others. The substrate can be non-porous, rigid, semi-rigid,etc. It can be formed into a well, a trench, a flat surface, etc. The substrate can include solids, such as siliceous substances such as silicon, glass, fused silica, quartz, and other solids such as plastics and polymers, such as polyacrylamide, polystyrene, polycarbonate, etc. Typically, the solid substrate is called a wafer from which individual chips are made (See the U.S. patents above which are incorporated herein by reference). As such, the pattern formed on the mask is projected onto the wafer to define which portions of the wafer are to be deprotected and which regions remain protected. See, for example, U.S. Pat. Nos. 5,593,839 and 5,571,639 which are hereby incorporated by reference in their entireties.
The lithographic or photochemical steps in the synthesis of nucleic acid arrays may be performed by contact printing or proximity printing using photomasks. For example, an emulsion or chrome-on-glass mask is placed in contact with the wafer, or nearly in contact with the wafer, and the wafer is illuminated through the mask by light having an appropriate wavelength. However, masks can be costly to make and use and are capable of being damaged or lost.
In many cases a different mask having a particular predetermined image pattern is used for each separate photomasking step, and synthesis of a wafer containing many chips requires a plurality of photomasking steps with different image patterns. For example, synthesis of an array of 20 mers typically requires approximately seventy photolithographic steps and related unique photomasks. So, using present photolithographic systems and methods, a plurality of different image pattern masks must be pre-generated and changed in the photolithographic system at each photomasking step. This plurality of different pattern masks adds lead time to the process and complexity and inefficiency to the photolithographic system and method. Further, contact printing using a mask can cause defects on the wafer so that some of the reaction sites are rendered defective. Thus, a photolithographic system and method that does not require such masks and obviates such difficulties would be generally useful in providing a more efficient and simplified lithographic process.
In view of the above, one advantage of the invention is providing an improved and simplified system and method for optical lithography.
Another advantage of the present invention is providing an optical lithography system and method that dynamically generates an image using a computer and reconfigurable light modulator.
A further advantage of the present invention is providing an optical lithography system and method that does not use photomasks.
A still further advantage of the present invention is providing an optical lithography system and method that uses computer generated electronic control signals and a spatial light modulator, without any photomask, to project a predetermined light pattern onto a surface of a substrate for the purposes of deprotecting various areas of a polymer array.
According to one aspect of the invention, polymer array synthesis is performed using a system without photomasks.
According to a second aspect of the invention, polymer array synthesis is performed using a system with a transmissive spatial light modulator and without a lens and photomask.
According to another aspect of the invention, a Direct Write System transmits image patterns to be formed on the surface of a substrate (e.g., a wafer). The image patterns are stored in a computer. The Direct Write System projects light patterns generated from the image patterns onto a surface of the substrate for light-directed polymer synthesis (e.g., oligonucleotide). The light patterns are generated by a spatial light modulator controlled by a computer, rather than being defined by a pattern on a photomask. Thus, in the Direct Write System each pixel is illuminated with an optical beam of suitable intensity and the imaging (printing) of an individual feature on a substrate is determined dynamically by computer control.
According to a further aspect of the invention, polymer array synthesis is accomplished using a class of devices known as spatial light modulators to define the image pattern of the polymer array to be deprotected.
An even further aspect of the present invention provides methods for synthesizing polymer arrays using spatial light modulators and the polymer arrays synthesized using the methods taught herein.
As can be appreciated by one skilled in the art, the invention is relevant to optical lithography in general, and more specifically to optical lithography for polymer array synthesis using photolithograpic processes. However, it is inherent that the invention is generally applicable to eliminating the need for a photomask in optical lithography.